Vibration / rattle at start up?

2013 Prius V
At some point this spring I started getting a vibration (more like a rattle) when I first start the car AFTER IT HAS SAT FOR A WHILE. When the car is warm, no noise. When the car switches to Hybrid, no noise. Noise is only there when the gas engine first starts. Car has 80K miles on it if that helps. Noise appears to only last until the gas engine is warm enough to switch to electric. No it is not the LOUD knocking sound you get when car was started for a short time then let to cool. Hard to tell but I believe the sound is 'up' not at the bottom of the engine compartment. I swear it is a loose something but I can not locate it... so far. Anyone have a similar noise? Thank you.

 

Just drove the car tonight after day of sitting... Noise is definitely there but maybe more of a 'buzz' than a real rattle. EGR circuit cleaning??? That I need help with.... ? What and where??? thank you.

 

Would it throw a code that could be read? So far I am getting no warning lights. Last trip to dealer the desk guy said any thing serious would code. He did listen to the car but by then it was warmed so not making the noise. I may eventually have to leave it overnight, with understanding it should be check immediately in the morning. Any chance this might be covered under my Certified Warranty or Emissions Warranty? Really do not need any extra bills right now.... Need extra cash for stash of toilet paper....

 

We still have some cold morning here but would extra minutes of lower mpg be a sign of the valve not working? I am talking about the 'mpg stacks' on the screen.